My guess so far is that a superposition is when a particle is described by two wavefunctions, which happen to be two eigenfunctions [itex]\psi_n[/itex]with the same (perhaps with different, {not sure there} ) eigenvalues En. Am I correct? If not, please enlighten me :)

I think "Linear Combination of Wavefunctions" was the term I was looking for, eh?

The term you are looking for is "linear combination of Energy eigenstates." For example, take the case of a particle in a 1D box of width L. The energy eigenfunctions are:

[tex]\phi_n = A sin(\frac{n \pi x}{L}) [/tex]

Where A is a normalization factor. Just because a particle is in this box, does not mean that it is one of the states, those are only the states with definite well defined energy. A particle could be in a super position of energy eigenstates:

[tex]\psi= B sin(\frac{\pi x}{L}) + C sin(\frac{2 \pi x}{L})[/tex]

where a condition on B and C is to normalize the wavefunction, as usual. Notice that the two states which are involved are the n = 1 state (the B term) and the n = 2 state (the C term). Now when we measure the energy of a particle in this state we do not know whether
you will get n = 1 or n = 2 but we can calculuate the probabiity of either!